A voice collected by a microphone may contain a noise component. When a collected voice contains a noise component, the voice may be hard to hear. Moreover, the noise component may reduce the recognition accuracy of the voice. Thus, a technique has been developed to suppress noise components by estimating, for each frequency, a noise component contained in a voice signal and subtracting the estimated noise component from the amplitude spectrum of the voice signal. In this technique, however, signal components may be also suppressed with noise components when the noise components are suppressed. This may lead to difficulty in hearing the original voice. To solve this problem, a technique has been proposed to compute a signal-to-noise ratio for each frequency, select a frequency band in which a signal component that can be recognized in a voice signal is represented by the signal-to-noise ratio, and determine a gain according to the signal-to-noise ratio of the selected frequency band (For example, see Japanese Laid-Open Patent Publication No. 2014-106494).